Motor control system for regulating tension



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ITIVEY'ICOTS'. ETne'sc'W- Hutton Raymond C. Locke Filed Aug. 2, 1950 E. W. HUTTON ET AL MOTOR CONTROL SYSTEM FOR REGULATING TENSION Feb. 3, 1953 Patented Feb. 3, 1953 MOTOR CONTROL SYSTEM FOR REGU- LATING TENSION Ernest W. Hutton, Scotia, and Raymond C. Locke,

Pattersonville, N. Y., assignors to General Electric Company, a corporation of New York Application August 2, 1950, Serial No. 177,170

3 Claims. E

This invention relates to knitting machines and more particularly to a new and improved apparatus for controlling the warp beam let-off in such machines.

In forming a knitted fabric in a conventional knitting machine a plurality of warp threads are drawn under tension from a Warp beam and passed over a tension member on their way to the knitting elements. The tension member generally comprises a rod carried on pivotally mounted arms which allow the rod to oscillate against biasing springs under variations in the tension of the warp threads. To maintain satisfactory tension in the threads, it is necessary that the unwinding or let-01f of the warp threads be suitably controlled. Heretofore the control of warp letoff has largely been through the use of me chanical devices, such for example, as clutches or brakes which control the rotation of the warp beam. While such mechanical devices have found widespread use, considerable trouble has been encountered due to variations in the triotional characteristics thereof, as influenced by such factors as humidity, temperature, and wear, which tend to produce shade marks, or alternate looseness and tightness, in the finished fabric. Moreover, the maintenance of mechanical devices to minimize such eiiects is generally troublesome and costly.

It is accordingly an object of this invention to provide a new and improved apparatus for controlling the let-off of warp threads in a knitting machine to maintain suitable average tension in the warp threads.

It is another object of the invention to provide apparatus of the type mentioned which is substantially independent of friction and wear and which, therefore, is highly consistent in operation, tending to reduce to a minimum the occurrence of shade marks and other imperfections in the finished fabric due to tension variations of the beam.

It is still another object of the invention to provide an apparatus of the type mentioned which is simple and reliable and which requires a minimum of maintenance.

In accordance with the invention the position of the tension rod in a knitting machine is sensed by the use of a rotary induction device, which may take the form of a dynamolectric machine of the self-synchronous type, coupled mechanically to the tension rod. In the art and hereinafter this rotary induction device will be referred to as a selsyn generator. Departure of the rod from a predetermined mean position pro- 8d vides error signals having respective phases responsive to the direction of the departure and amplitude responsive to the amount of departure. Because the allowable variation in the mean position of the tension rod is small in comparison with its total motion occurring during each knitting course, the signals derived from the sensing device are impressed on a discriminating circuit wherein signals of opposite phase are integrated to secure an error signal responsive to the average departure of the tension rod from the mean position. The integrating circuit is employed such that the error signal is not a percentage change in the total tension rod movement but is. actually a signal having an amplitude that is proportional to the departure of the actual mean position of the tension rod from the desired average mean position. The integrated error signal is employed through a power circuit to vary the speed of a dynamoelectric machine which is coupled through suitable gearing to the warp beam. Rotation of the latter, which is effected by tension in the warp threads, is controlled by releasing action of the dynamoelectric machine through the associated gearing.

For further objects and advantages and for a better understanding of our invention, attention is now directed to the following description and accompanying drawing and also to the appended claims.

In the single figure of the drawing there is shown a schematic diagram of a Warp thread letoff control for a knitting machine constructed in accordance with my invention.

Referring to the drawing, there is shown in the upper portion thereof certain elements of a conventional knitting machine, the various elements shown being assumed to be commonly mounted with. respect to the main frame (not shown) of the knitting machine. Warp threads I are drawn by action of knitting elements 2 from warp beam 3 in contact with a guide rod 4 and a tension member 5. The knitted fabric is taken up on a roller 6. Knitting elements 2 are actuated by a cam mechanism l' which is driven by a suitable electric motor 8.

Tension member 5 comprises a tension bar 9 extending across the full width of the knitting machine. Rod 9, over which threads I pass, is supported at each end by an arm ID pivotally mounted at II on the frame of the machine. An opening is provided in the free end portion of arm 10 to permit passage therethrough of a rod i2 which is rigidly mounted at l3 on the frame of the machine. Rod I2 carries a compression spring I4 which acts between arm I9 and nuts I5 threadedly engaged on rod I2. Spring l4 resists downward motion of tension rod 9 under the tension of warp threads I, and thus the degree of tension in threads i is adjustable by movement of nuts I5. It will be apparent that oscillatory motion of arm I0 about pivot II is indicative of variation in the tension in threads I. The motion of arm I9 is sensed by th use of a selsyn generator I6 having its rotor mechanically connected to arm If] and arranged to be rotated in response to motion thereof. Selsyn I6 is provided with a primary winding H, which is connected across a source of alternating power, and with a secondary winding I8. The phase of the voltage induced across winding I8 by the current in winding I! is indicative of the direction of winding I8 with respect to winding I7, and the amplitude of the voltage induced across winding I 8 is proportional to the cosine of the angle between the main axes of the coils. Consequently, the amplitude and the phase of the voltage induced across winding I8 is dependent upon the position of arm I0 and hence upon the tension in threads I.

Let-off of threads I from warp beam 3 is accomplished by the use of a dynamoelectric machine or motor I9 coupled to beam 3 through a suitable gear train 20. The latter includes a worm gear 2|, and the entire gear train is preferably constructed so that reverse driving does not occur therethrough. Beam 3 i effectively locked and a releasing action must be supplied by motor I9 through gear train to effect rotation of beam 3. It will be apparent, therefore, that the let-oiT of threads I and the tension therein may be regulated by controlling the rotation of motor I9. Motor I9 is represented herein as being of the single phase alternating current type having a pair of windings 22 and utilizing a capacitor 24 to shift the phase of the current in one of the windings to provide a starting torque. While such a machine is particularly suited to use in a knitting machine due to the absence of brushes, which even in a lint-laden atmosphere require a minimum of maintenance, it will be understood that other types of motors may be employed in connection with the invention. Motor I9 is serially connected with a primary winding of a transformer 24 across a source of alternating power.

Attention is now directed to the control and power circuits which utilize the signals provided by selsyn generator IE to control the rotation of motor I9 to maintain substantially constant average tension in threads I. Because the voltage across windings 22 determines the speed of rotation of motor I9 the impedance of the winding of transformer 24 controls the speed of rotation of motor I9. An electron discharge device 25 having electrodes located in an ionizable medium and hereinafter referred to as a thyratron is connected across the secondary winding of transformer 24 such that conduction of thyratron 25 short circuits the secondary winding and decreases the impedance of the primary winding of transformer 24. Consequently, firing of device 25 increases the speed of rotation of motor I9. The anode of thyratron 25 being connected to the secondary winding of transformer 24 is positive during one half cycle of the alternating source and negative during the succeeding half cycle. Two conditions must be fulfilled at the same instant to cause thyratron 25 to fire. One of these conditions is that the anode be positive with respect to the cathode and the other condition is that the control electrode have a voltage which is higher than the critical voltage for the articular thyratron employed. As is well known in the art of control, an alternating voltage on the control electrode which is 99 degrees out of phase lagging with the anode voltage of a thyratron causes the thyratron to fire for approximately one-quarter of a cycle. A direct voltage component may be added, and, as it is increased in a positive sense, the firin time of the thyratron approaches a half cycle. As the direct voltage of the control electrode is increased in a negative sense, the firing time of the thyratron is decreased to less than a quarter of a cycle.

To supply the degree phase shifted voltage on the control electrode, we provide a transformer 29 having its primary winding connected to the same source of alternating power as is the primary winding of transformer 24 and having its secondary winding connected in a closed loop with a variable resistor Eland a capacitor 28. Because the capacitive reactance of capacitor 23 is chosen much lower than the resistance of resistor 21, the voltage drop across capacitor 28 lags the current through the secondary winding of transformer 26 by substantially 9!) electrical degrees. Therefore, the alternating voltage signal supplied to the control electrode of thyratron 2 5 lags the anode-to-cathode voltage by 90 degrees. With no direct voltage bias on the control electrode of device 25, the tube fires during approximately one-quarter of a cycle and motor I!) rotates at a speed which is dependent upon this time of conduction of thyratron 25. It should be understood that other factors may effect the speed of motor I9, but in a closed system such as this, these factors become negligible. Because of the flywheel effect of the armature of motor I9, the shaft of the motor does not turn during only one portion of the cycle but turns continuously, and the rate at which it turns is dependent upon the impedance of the primary winding of transformer 24 averaged during one cycle. To derive the direct voltage to control the time of firing of thyratron 25 from the winding I 8 of selsyn generator Iii, we employ a pair of triode electron discharge devices 29 and 39 connected backto-back. For convenience of installation, these devices should preferably be enclosed in one en velope such, for example, as in a GSN'Z electron tube. To prevent triodes 29 and 39 from operating as a ring oscillator, a capacitor 3! is connected between the anode of triode 39 and the anode of triode 29 and attenuating resistors 92 and 33 are connected in the anode circuits of the two tubes. The anodes of triodes 29 and 39 are alternately positive because the secondary winding of transformer 25 is used to supply a the anode voltage for the two triodes. A loading resistor 39 supplies a direct current path between the tubes and their source of voltage, the secondary winding of transformer .26. lhe energizing coil of relay K1 is connected in the drive motor circuit (not shown) of the knitting machine so that during operation of the drive motor contacts 35 are bridged by the movable contact Because contacts 35 and 3511 are closed, a secondary winding 35 of a transformer 31 is electrically connected between the cathode and control electrode of triode 29. Another secondary winding 38 of transformer 37 is electrically connected between the control electrode and cathode of triode 39.

As isknown in the art, heating a metal inamm creases the static potentialbetween that metal and some other point which is unheated. It is this phenomena that is used to obtain the grid bias voltage for vacuum tubes 29 and 30. To sustain this bias voltage which is of the order of a few volts, high ohmic resistors 39 and 40 are connected in the grid circuits of triodes 29 and respectively, and these resistors are bypassed by capacitors 4| and 42 to compensate for the inner electrode capacitances of the tubes. time constants of resistors 39 and capacitor 4| and resistor and capacitor 42 are chosen low enough that unblocking occurs before the control equipment loses control of motor 19.

Primary winding 43 of transformer 31 is directly connected across winding l8 such that any voltage magnetically induced in winding l8 by the current in winding 11 appears across primary winding 43 and hence on windings 36 and 38.

Because selsyn winding I1. is connected across the same source of alternating potential as are the anodes of triodes 29 and 30, the alternating voltage signal on the control electrodes of these triodes will either be in phase or 180 out of phase with their respective anode voltages, depending upon the position of selsyn winding l8 with respect to selsyn winding 11. When winding I8 is perpendicular to winding IT as is shown in the drawing, no voltage is induced across winding 18 and consequently, no alternating voltage appears on the control electrodes of triodes 29 and 39. However, as the tension in threads I varies, winding I8 is rotated with respect to winding l1 and a voltage appears across winding 43. The phase of this voltage is such that either the control electrode and the anode of triode 29 are in phase or the control electrode and the anode of triode 39 are in phase. If the control electrode and the anode of triode 39 are in phase, triode 38 will conduct for a, portion of a half-cycle which depends upon the amplitude. of the signal on the control electrode and, consequently, upon the amplitude of the voltage induced across winding l8. When triode 39 conducts, the control electrode of thyratron 25 is lowered in voltage with respect to its cathode while when triode 29 conducts the voltage of the control electrode of thyratron 25 is increased with respect to its cathode. As a result, thyratron 25 fires more or less than a quarter of a cycle depending upon the position of selsyn winding IS with respect to selsyn winding l1.

It will be understood that during normal knitting operation tension bar 9 bobs up and down rather than remaining in. a single position. Because these bobs or excursions are necessary for the proper operation of the machine, it is. undesirable that the controller follow these excursions, and, consequently, only the difference between the excursions in one direction and those. in the other direction is used. to produce an error signal. The difference between these excursions in opposite directions is characterized by one of the tubes conducting more heavily than the other. Capacitor 28 integrates the signals from triode 29 and 3E! and balances them out such that only the average difference signal appears on the control electrode of thyratron 25 to control the speed of rotation of motor 19. Capacitor 2.8 is also, employed, as a filter such that. it and the flywheel action of motor l9 heretoforementioned cooperate to provide smooth running of warp beam 3. A capacitor 44 parallels winding 36 to diminish the, effects, of pick-up in the various leads of the device. A coupling circuit compris- The 7 point is connected to the control electrode of from one machine to another.

thyratron. 25.

A coast-stop knitting machine, when stopped, coasts through the entire speed range to standstill. In addition, the amount of coast varies from time to time on a machine and is different As tension bar 9 moves through its excursions at a slower and slower rate during coasting, the control device,

rather than following the average difference between up and down excursions, gradually starts following the peaks of the up excursions. This would result in a change in tension but for the compensating circuit which functions as follows If the controller is to follow the machine as it coasts to a standstill, it is necessary that the signal characteristics of the voltage applied to fire thyratron 25 be such as to match the slow-down characteristics of the knitting machine. To supply a signal to the control electrode of triode 29 which will cause warp beam 3 to slow down in direct relation to the slow-down of the machine,

a bias is provided on the control electrode of triode 29 which increases in negative value as the machine slows down. To obtain this negative bias, an alternating voltage signal is supplied from the source of alternating power through a transformer 4? across a variable potentiometer 48. The signal from potentiometer 48 is supplied through a potentiometer 49 to the control electrode of triode 29. When the power to the drive motor of the knitting machine is cut off,

relay K1 is (fie-energized, opening contacts 35 such that the negative polarized bias is applied across diode 58 from transformer 41 in series with the positive bias from tachometer 5i and circuit components 52, 49 and 53. If the sum of these biases is positive, the diode limits the voltage to essentially zero and no change occurs to the bias on triode 29. As the tachometer speed decreases, the positive bias decreases in accordance with the machine coasting, and when it becomes less than the uegative bias from transformer 41 the difference appears as a negative bias on triode 29. This increasing negative bias causes the control of motor 22 to be such that the average position of the tension rod is maintained. This is explained in more detail as follows:

A direct current tachometer 5!, which in reality is a direct current generator, has its shaft connected to the shaft of cam I such that as the speed of the knitting machine decreases the voltage across the tachometer 5! decreases. The voltage from tachometer 51 is coupled through a choke coil 52 and a capacitor 53 to potentiometer 49. Choke coil 52 and capacitor 53 prevent interference in the form of commutation ripple and high frequency pick-up with the operation of the control circuit. A resistor 54, which is connected in parallel relation with a capacitor 55, is used to limit the current in diode 55. Capacitor 55 is connected in this circuit to reduce pick-up in the leads of the coast circuit.

When the machine is running at full speed and the drive motors are disconnected to stop the machine, contacts 35 are open-circuited and the voltage generated by tachometer 5! is superimposed on the alternating voltage from transformer t! which results in a positive bias to 'of tube 29 is positive with respect to the cathode oftube 29.) Diode 59, however, acts as a clipper tube to a positive bias reducing it to a negligible value. As the machine slows down, the voltage of tachometer 5| decreases so the sum of its voltage and that portion of transformer 41 voltage occurring during the time period the anode of tube 29 is positive becomes zero and then increasingly negative. At the time this voltage becomes zero or negative, diode 5B ceases to function and this bias becomes a take-over bias which reduces the speed of motor I 9 essentially in conformance with the slowing down to standstill of the machine. Because different types of fabric cause different amounts of excursion of tension rod 9, the normal control circuitloses control at different times after the machine has been cut off. This coasting control circuit is designed such that when the main control circuit loses control because of the slow rate at which bar 9 moves up and down and motor l9 would ordinarily begin to follow these excursions, the signal from tachometer 5! takes over control of triode 29 and, hence, control of motor l9. As previously mentioned, for different materials the time at which the system loses control differs, and, consequently, a tachometer is chosen such that the voltage derived therefrom is as great, if not greater, than that required for any type of knitting. Limiting rectifier is used, therefore, such that when the bias on the control electrodes of tubes 29 and 30 reaches zero, which is an indication that control is lost, the bias is supplied to the grid by tachometer 5| and increases in value as the machine slows down.

In hand operation, that is, when the drive motors are disconnected and the machine is turned over by hand such, for example, as when broken threads are being replaced or broken needles are being replaced, potentiometer 48 is used to supply the necessary bias to triode 29 to cause the control, although following at this slow speed the peaks of the up excursion, to still maintain the average position of the tension bar the same as when running in normal knitting operation.

When the machine is first turned on and needles 2 begin to knit, the signal derived through selsyn i6 and triode amplifiers 29 and 3c is not great enough to supply the necessary accelerating torque for motor 59 until a large error signal is present. Consequently, a start mark or an imperfection appears in the fabric during that time interval between which the drive motor is started and the control motor picks up to speed. To prevent start marks in the fabric, we employ what we call a preconditioning circuit which supplies the necessary accelerating torque to motor is when the drive motor is first turned on. While the machine is idle but the control circuit is in operation, contacts 57 of a relay K2, which has its energizing coil connected in the main drive circuit, are short-circuited such that capacitor 58, which is serially connected with a resistor 5d, a resistor 68, and a diode rectifier 6i across the secondary winding of transformer 26, is charged up. Energizing of relay K2 closes contacts 62 which causes capacitor 58 to discharge to capacitor :23 which supplies a negative bias to the control electrode of triode 36 to pre vent it from conducting. Consequently, only triode 29 can conduct, and it supplies a high positive voltage to the control electrode of thyratron 25 which greatly reduces the impedance in the circuit of motor l9 imparting a high starting torque to the motor. This means of supplying a high signal to the control electrode of thyratron 25 is possible because capacitor 28, which, as' heretofore mentioned, normally balances the plus excursions against the minus excursions, is, in this case, supplied with no signal from triode 3% which in normal operation supplies the negative voltage to the control electrode of thyra tron 25. A variable resistor 64 is connected across capacitor 63 to make adjustable the actual bias and time duration of this bias on the control electrode of triode 30.

- This controller is equipped to control a machine which is in normal operation, which is in hand operation, or which is in coasting operation. Some knitting machines are equipped with brakes for suddenly stopping the machine and the action of the knitting needles. This controller may be used with such a machine by merely short circuiting contacts 35. It should be understood that tachometer 5| and its associated circuit elements'may be'eliminated in a controller adapted for such a machine.

This machine has the advantage over prior art types in that it measures the difference between the desired average position of tension rod 9 and the true average position. Consequently, an increased excursion of bar 9 because of the knitting of a particular type of fabric does not decrease the accuracy of control. Having once been adapted for a particular machine by the adjustment of the various resistors this controller is fully automatic. Of course, for different types of fabric those resistors hereinbefore mentioned may be adjusted. Because of the averaging effect of capacitor 28 and consequently because a percentage difference is not used as the control signal, this system has both high sensitivity and good stability. The sensitivity of this machine is such that it detects and corrects for excursions of tension bar '9 which are not visibleto the naked eye.

While this invention has been described by particular embodiments thereof, it will be understood that those skilled in the art may make many changes and modifications without departing from this invention. Therefore, the appended claims are intended to cover all such changes and modifications which fall within the true spirit and scope of this invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. Means for controlling a dynamoelectric machine in response to changes in the average position of a movable member, comprising a rotary induction device having a stationary winding and a rotary winding, said rotary winding being rotated by movement of said movable member, said stationary winding being connected to a source of alternating power, a discriminating circuit being supplied with a voltage signal from said rotary winding, an integrating circuit for integrating an output voltage of said discriminating circuit, an electron discharge device being provided with'an anode, a cathode, and a control electrode, said anode, cathode, and control electrode being located in an ionizable medium, the output voltage from said integrating circuit being supplied between said control electrode and said cathode, means supplying an alternating voltage tosaid anode and an alternating voltage to said control electrode, the voltage of said control electrode lagging the voltage of said anode by ninety electrical degrees, and a transformer having a secondary-winding connected between said anode and said cathode and a primary winding serially connected with the input terminals of said dynamoelectric machine between the terminals of a source of alternating power.

2. Means for controlling the speed of rotation of a dynamoelectric machine in response to changes in the phase and amplitude of an a1ternating current signal comprising a transformer having a first winding serially connected between the input of said machine and a source of alternating power; means for varying the impedance of said first winding including an electron discharge device having an anode, a cathode, and a. control electrode, said anode and said cathode being connected to different points on a second winding of said transformer, means for supplying said control electrode with a lagging voltage with respect to the voltage on said anode including a capacitor, a second transformer having a first winding connected to said source of alternating power and a second winding connected with said capacitor to said control electrode, means for varying the direct potential of said control electrode in response to said alternating current signal including a third transformer having a first, a second, and a third winding, said first winding being supplied with said signal, a first and second electronic valve each being provided with an anode, a cathode, and a control electrode, the anode of said first valve being electrically connected to the cathode of said second valve and the anode of said second valve being connected to the cathode of said first valve, said second and third windings of said third transformer being connected to the control electrodes of said first and second valves respectively, and said first and second valves being connected across said capacitor.

3. Means controlling a dynamoelectric machine in response to changes in the average position of a movable member the position of which is dependent upon the load on said member, comprising a rotary induction device having a stationary and a rotary winding, said rotary winding being rotated by movement of said movable member, said stationary winding being connected to a source of alternating voltage; means for producing a direct voltage signal in response to rotation of said rotary winding, said last-mentioned means comprising a first and a second a electron discharge device connected in back-toback relation as a discriminator circuit, electrical connections for supplying the anodes and the control electrodes of said devices with an alternating voltage from a source of alternating voltage, means supplying the anodes and the control electrodes of said discharge devices with an alternating voltage, and an integrating circuit including a capacitor connected to be supplied with an output voltage from said discriminating circuit; means maintaining control of said dyna nloelectric machine during periods when the shaft of said motor is decelerating, comprising a tachometer generator being mounted to produce a voltage dependent upon the speed of rotation of the shaft of said motor, a source of alternating voltage and said generator being serially connected between a control electrode and a cathode of said first discharge device to supply a bias voltage thereto which is dependent upon the speed of said motor, a unilateral impedance device being connected ll atween the control electrode and the cathode of said first device to prevent the voltage of the control electrode from being positive with respect to that of the oathode, and a contact device connected to shortcircuit said impedance device in response to application of power to said motor; and means con trolling said dynamoelectric machine in response to a voltage signal from said integrating circuit, comprising a transformer having a first and a second winding, said first winding being serially connected between a source of alternating voltage and input terminals of said dynamoelectric machine, an electron discharge device being provided with an anode, a cathode and a control electrode which are located in an ionizable medium, electrical connections supplying said second winding with an outp t voltage from said discharge device and electrical connections for supplying said control electrode with the output voltage of said integrating circuit.

. ERNEST W. HU'ITON.

RAYMOND C. LOCKE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1963,246 Purington June 19, 1934 2,223,718 Cook Dec. 3, 1940 2,544,467 Michel Mar. 6, 1951 

